1. Field of the Invention
The present invention relates to an apparatus for storing image signals, and more particularly to an apparatus and a method of storing image signals suited for storing high definition image signals and for effectively utilizing an interface and a transmission medium of low definition image signals. The invention is further related to a data transform method and apparatus which are adapted to the cases where it is not allowed to transmit particular signal levels.
2. Background Art
An apparatus for compressing image signals has been disclosed in, for example, Japanese Patent Laid-Open No. 107013/1992. Apparatuses that compress image signals and then regenerate for magnetic recording have also been disclosed in Japanese Patent Laid-Open Nos. 137823/1992 and 170879/1992.
The above prior arts teach technology for compressing image signals so that they can be transmitted over a predetermined transmission medium and technology related to an apparatus that records image signals by compressing them. Nothing, however, is disclosed in relation to technology for effectively utilizing interfaces and transmission medium of low definition image signals.
In a digital signal processing circuit for transmitting image signals, an 8-bit word is usually treated as a unit. In a digital VTR of D-2 format, for instance, the ordinary TV signals are sampled with a frequency four times as great as that of a color sub-carrier wave, and the sampled values are recorded as 8-bit binary data.
However, in the case of a digital image apparatus as represented by a digital VTR of D-2 format, it has been standardized not to use 00(H) or FF(H) (H indicates hexadecimal notation). With the D-2 VTR, 00(H) has a value far smaller than a sync tip and does not almost appear as an image signal. Therefore, the D-2 VTR is almost not affected even when 00(H) is replaced by 01(H). Similarly, FF(H) is larger than a value of an ordinary image signal and scarcely appear as an image signal. Therefore, the VTL is not quite affected even when FF(H) is replaced by FE(H).
However, when it is attempted to transmit a digital data instead of an image signal, even 00(H) and FF(H) which have the same meanings as other data are not allowed to be replaced by other data. Moreover, an apparatus that handles image signals effects the processing with a synchronizing signal as a reference. When values smaller than a predetermined value are continuously generated, therefore such values are treated as synchronizing signals. Therefore, the general digital data could not be transmitted over a transmission medium of transmitting image signals.